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"""Add indexes for CASCADE deletes on task_instance.

Some databases don't add indexes on the FK columns so we have to add them for performance on CASCADE deletes.

Revision ID: f5fcbda3e651
Revises: 3c94c427fdf6
Create Date: 2022-06-15 18:04:54.081789

"""

from __future__ import annotations

from alembic import context, op
from sqlalchemy import text

# revision identifiers, used by Alembic.
revision = "f5fcbda3e651"
down_revision = "3c94c427fdf6"
branch_labels = None
depends_on = None
airflow_version = "2.3.3"


def _mysql_tables_where_indexes_already_present(conn):
    """
    Check if certain indexes already exist.

    If user downgraded and is upgrading again, we have to check for existing
    indexes on mysql because we can't (and don't) drop them as part of the
    downgrade.
    """
    to_check = [
        ("xcom", "idx_xcom_task_instance"),
        ("task_reschedule", "idx_task_reschedule_dag_run"),
        ("task_fail", "idx_task_fail_task_instance"),
    ]
    tables = set()
    for tbl, idx in to_check:
        if conn.execute(text(f"show indexes from {tbl} where Key_name = '{idx}'")).first():
            tables.add(tbl)
    return tables


def upgrade():
    """Apply Add indexes for CASCADE deletes."""
    conn = op.get_bind()
    tables_to_skip = set()

    # mysql requires indexes for FKs, so adding had the effect of renaming, and we cannot remove.
    if conn.dialect.name == "mysql" and not context.is_offline_mode():
        tables_to_skip.update(_mysql_tables_where_indexes_already_present(conn))

    if "task_fail" not in tables_to_skip:
        with op.batch_alter_table("task_fail", schema=None) as batch_op:
            batch_op.create_index("idx_task_fail_task_instance", ["dag_id", "task_id", "run_id", "map_index"])

    if "task_reschedule" not in tables_to_skip:
        with op.batch_alter_table("task_reschedule", schema=None) as batch_op:
            batch_op.create_index("idx_task_reschedule_dag_run", ["dag_id", "run_id"])

    if "xcom" not in tables_to_skip:
        with op.batch_alter_table("xcom", schema=None) as batch_op:
            batch_op.create_index("idx_xcom_task_instance", ["dag_id", "task_id", "run_id", "map_index"])


def downgrade():
    """Unapply Add indexes for CASCADE deletes."""
    conn = op.get_bind()

    # mysql requires indexes for FKs, so adding had the effect of renaming, and we cannot remove.
    if conn.dialect.name == "mysql":
        return

    with op.batch_alter_table("xcom", schema=None) as batch_op:
        batch_op.drop_index("idx_xcom_task_instance")

    with op.batch_alter_table("task_reschedule", schema=None) as batch_op:
        batch_op.drop_index("idx_task_reschedule_dag_run")

    with op.batch_alter_table("task_fail", schema=None) as batch_op:
        batch_op.drop_index("idx_task_fail_task_instance")
